1. Field of the Invention
The present invention relates generally to a drive circuit for a flat panel display device and a driving method using the drive circuit and, more particularly, to an Active Matrix Organic Light-Emitting Diode (AMOLED) drive circuit using transient current feedback and an active matrix driving method using the AMOLD drive circuit, which, when an AMOLED display device is driven in a current mode, can overcome a decrease in driving speed, which is caused by charging or discharging due to the parasitic capacitance of data lines, using current feedback based on the detection of transient charging current, and which divide the data lines of a display panel into even data lines and odd data lines and alternately perform a data write operation on the even data lines and the odd data lines, thus reducing the number of channels of a drive Integrated Circuit (IC).
2. Description of the Related Art
An Organic Electroluminescent (OEL) device, which is a new type of flat display device, is a self-emitting device, so that it has an excellent viewing angle and contrast ratio compared to a Liquid Crystal display (LCD) device. Furthermore, the OEL device does not require a backlight, and thus it can be implemented to have a light weight and a thin size and also has an advantage in power consumption.
Furthermore, the OEL device has many advantages in that it can operate at a low Direct Current (DC) voltage, has a fast response speed, is robust to external impact because it is formed of solid-state components throughout, and can be used in a wide temperature range. In particular, the OEL device has an advantage in that the manufacturing cost is low. Such an OEL device is called an Organic Light Emitting Diode (OLED).
Unlike an LCD device or a Plasma Display Panel (PDP) device, the OEL device is manufactured through a very simple process, and thus the process can be sufficiently performed using only deposition and encapsulation equipment.
Particularly, in an active matrix scheme, a storage capacitor (CST) is charged to a voltage for controlling current, which is applied to pixels, so that the charged voltage can be applied until a subsequent frame signal is applied, therefore the pixels can be continuously driven during one frame period regardless of the number of gate lines.
Accordingly, the active matrix scheme can achieve the same brightness even when a low current is applied, therefore it has advantages in that it can be manufactured to have low power consumption, high definition, and a large size.
Conventional display devices, each of which uses the flat panel display element having the above-described characteristics, are described with reference to U.S. Pat. Nos. 6,433,488 and 6,809,706 below.
FIG. 1 is a representative circuit diagram disclosed in U.S. Pat. No. 6,433,488. A drive transistor 21 and an OLED 1, which is a light-emitting element, are connected in series to each other, so that the same current flows through the drive transistor 21 and the OLED 1 in a data writing period. This current is called drive current, and is transferred to one input terminal of a current comparator 6 through a switching transistor 53.
The current comparator 6 has two input terminals and one output terminal, compares drive current with reference current, and outputs a voltage, corresponding to the result of the comparison, to a data input terminal to make the value of the drive current the same as the reference current. This output voltage is input to the gate of the drive transistor 21 through a switching transistor 22.
FIG. 2 is a circuit diagram showing the circuit of FIG. 1 in detail. The core circuits of the drive circuit are circuits for implementing a current comparison circuit and converting the output of the current comparison circuit into voltage. That is, the current comparator 6 includes a current mirror REF for generating reference current, another current mirror DRV for generating drive current, and the current comparison circuit for comparing the outputs of the current mirrors REF and DRV and outputting a voltage.
As described above, the output of the current comparator 6 is input to the gate of the drive transistor 21 through the switching transistor 22.
The operation of FIGS. 1 and 2 has been described based on the signal paths formed during the data writing period.
FIG. 3 is a representative circuit diagram disclosed in U.S. Pat. No. 6,809,706. A drive transistor Tr2 and a light-emitting element 1 are connected in series to each other. A differential amplifier 25 is automatically controlled by the voltage of the anode of the light-emitting element 1, that is, a node J, so that the time-varying characteristic of a drive transistor Tr2 and spatial characteristic distribution in a panel can be overcome, therefore uniformity in brightness of a screen can be achieved.
The voltage of the node J follows the voltage of the reference input terminal 11 the differential amplifier 25 by the operation of the differential amplifier 25 and a feedback operation. Accordingly, drive current, which corresponds to the voltage of the reference input terminal 11 of the differential amplifier 25, flows through the drive transistor Tr2 and the light-emitting element 1.
In this case, under the assumption that the above-described operation is performed in the data writing period, the signal of a scan line 14 enters an enabled state, and thus all signal paths are connected to each other. In this state, as the differential amplifier 25 operates, the gate voltage of the drive transistor Tr2 is automatically controlled such that current, which is generated by the voltage applied to the anode of the light-emitting element 1, that is, the node J, flows through the drive transistor Tr2.
FIG. 4 is a circuit diagram showing a drive circuit using voltage feedback, which is disclosed in a paper entitled “New Driving Method for a-Si AMOLED Displays Based on Voltage Feedback,” published in the Society for Information Display (SID) 2005.
The drive circuit substantially employs the considerable part of the principle disclosed in U.S. Pat. No. 6,809,706. An input voltage Vin is continuously compared with feedback voltage VF through a feedback operation. When a voltage applied to a resistor RF is equal to the input voltage Vin, current through the resistor RF is determined by Vin/RF.
The gate voltage of a drive transistor T1 is automatically set by a differential amplifier (external driver) such that the current flows through the drive transistor T1 connected in series with the resistor RF. That is, the drive circuit allows current, which flows through an OLED, to be determined by Vin/RF, so that it can transfer data current, independent of the drive transistor T1, to the drive transistor T1 of a pixel circuit.
In this case, under the assumption that the above-described operation is also performed in the data writing period, the signal of a select line enters an enabled state, and thus all signal paths are connected to each other. The drive circuit has been described with the assumption that the distribution and time-varying characteristic of resistor RF are better than those of the drive transistor T1. This assumption is true in reality.
The drive circuit of FIG. 5 is a circuit that is implemented so as to be applied as a simpler pixel circuit when applying the drive circuit of FIG. 4 to a display panel. When implemented as described above, the drive circuit has a structure in which one resistor RF is connected to one data line, so that the characteristics of that data line can be uniformly controlled.
The drive circuit of FIG. 6 is a circuit that is implemented so as to perform a low current data write operation, in which the drive circuit of FIG. 5 causes a problem.
In the drive circuits of FIGS. 4 and 5, the input voltage Vin must have a very small value, or the resistor RF must have a very large value in order to generate extremely low current. This causes a problem that cannot be overcome in practice. Accordingly, another drive circuit is required to realize data writing using extremely low current. FIG. 6 shows a circuit structure that has been proposed as an alternative for achieving this purpose.
In the drive circuit (U.S. Pat. No. 6,433,488) of FIGS. 1 and 2, described above, parasitic capacitance is generated by a data line drive current input terminal and the current comparator, thus there is a problem in that it is difficult to ensure feedback loop stability due to the capacitance.
In particular, the drive circuit of FIG. 2 is a circuit which is impossible to operate. The reason for this is because the drain of the transistor N3 of the current comparator is connected to a voltage source Vpp, and thus the feedback loop of the drive circuit is incomplete.
Furthermore, in the drive circuit, the accuracy of the data drive current is largely limited. The reason for this is because the current mirrors are simple mirrors. Another problem, in addition to the above-described problems, occurs in that two lines, which are electrically connected to a drive chip, are required to drive a single data line on a display panel (one line forms a drive current path, and the other forms a data line path).
Furthermore, from the point of view of the drive chip, the degree of integration is generally determined by the distance between data channels. If the drive chip requires two electrical paths to drive a single data line, the degree of integration thereof necessarily becomes lower than that of a typical drive chip.
Furthermore, in order to normally implement the drive circuit of FIG. 3, there must not be any characteristic difference between the pixels of OLED elements, which are light-emitting elements, and there must not be any time-varying characteristic. However, the OLED elements that form respective pixels generally exhibit differences in characteristics therebetween. In particular, the OLED elements generally exhibit excessive variation in characteristics as the duration of use thereof increases. The variation in the characteristics depending on the duration of use thereof may cause a situation in which it is difficult to smoothly operate the drive circuit of FIG. 3.
The drive circuit of FIG. 4 can guarantee the uniformity of display only when resistors are formed in respective pixel circuits and have considerable matching characteristics therebetween.
However, the resistors are generally implemented by controlling the doping density and geometrical shape of polycrystalline silicon. In the resistors that are made through two processes and using the feature of the material thereof, considerable matching characteristics cannot be acquired, and the matching characteristics cannot be ensured as the resistance values thereof increase. Particularly, in the case where resistors must be implemented to correspond to respective pixel circuits, as in FIG. 4, the distribution thereof may be not smaller than that of drive Thin Film Transistors (TFTs).
The drive circuit of FIG. 5 has a structure in which the number of transistors, which is the cause of the problems in the drive circuit of FIG. 4, is greatly reduced. Furthermore the drive circuit has a structure in which one resistor is disposed to correspond to each data line, and pixel circuits, which share the same data line, uses the resistor. However, although, in the drive circuit of FIG. 5, the number of pixel circuits is greatly reduced in contrast to the case of FIG. 4, it cannot be expected that conditions necessary to match the resistors of respective data lines will be decreased or greatly improved.
As the common feature of the conventional drive circuits, shown in FIGS. 1 to 6, one data line and one sensing line are required to drive one data line, and an economical circuit structure and driving method are not used from the point of view of the drive chip.